There are various local ordinances which prohibit trash-picking for various reasons, such as health and safety, but the broad legal principle as held by the SCOTUS in California v. Greenwood
http://caselaw.lp.findlaw.com/scripts/getcase.pl?navby=search&court=US&case=/data/us/486/35.html is that trash placed in a publicly accessible location with no reasonable expectation of privacy (i.e. outside the "curtilage of a home") is free for anyone's perusal, and it would seem, taking.
Obviously the facts in the situation being discussed in this thread are not identical to those in Greenwood. Greenwood was about fourth amendment searches, and the "reasonable expectation of privacy" in one's trash, not precisely about the property rights to one's trash. Also, here in the gun-liberation case, the officers had special access to items which were not in a publicly-accessible location.
But the key is that once the items were headed for disposal, they were no longer the same type of property as they would be if simply locked-up in the evidence room. Greenwood implies that homeowners effectively forfeit their property rights to garbage placed for disposal, by discussing without condemnation the taking of trash by others. eg. "It is common knowledge that plastic garbage bags left on or at the side of a public street are readily accessible to animals, children, scavengers, snoops, and other members of the public. Moreover, respondents placed their refuse at the curb for the express purpose of conveying it to a third party, the trash collector, who might himself have sorted through respondents' trash or permitted others, such as the police, to do so." [footnotes and citation omitted]
The officers were disobedient to their employer, but they didn't take another person's or organization's property in the same sense as simply walking off with the office stapler. Hence the difficulty, perhaps impossibility, of demonstrating "larceny" on the part of the officers. Personally, I have trouble blaming them under the circumstances, but I must admit they did wrong by their employer. Even so, it wasn't larceny, just plain disobedience.
Incidentally, if you have any interest in the fourth amendment (and who on this second amendment-oriented board doesn't) Greenwood is excellent reading, as it also references some other key cases on the subject of the "reasonable expectation of privacy." Not many people realize that there's no reasonable expectation of privacy inside a home under a skylight, for instance, but that's the unstated implication of California v. Ciraolo. Food for thought.
-twency
By the way, I agree that this was a victimless "crime", but that doesn't necessarily make it right.